Add validation to ensure FastlyPurgeModule is installed

Check if the FastlyPurgeModule class exists during construction and throw a LogicException if it is missing. This prevents misconfigurations and ensures the required dependency is installed.

src/Cdn/FastlyModule.php

@@ -7,8 +7,11 @@
 use BEAR\FastlyModule\FastlyPurgeModule;
 use BEAR\QueryRepository\CdnCacheControlHeaderSetterInterface;
 use BEAR\QueryRepository\PurgerInterface;
+use LogicException;
 use Ray\Di\AbstractModule;
 
+use function class_exists;
+
 final class FastlyModule extends AbstractModule
 {
     public function __construct(
@@ -17,6 +20,10 @@ public function __construct(
         AbstractModule|null $module = null,
     ) {
         parent::__construct($module);
+
+        if (! class_exists(FastlyPurgeModule::class)) {
+            throw new LogicException('Install bear/fastly-module'); // @codeCoverageIgnore
+        }
     }
 
     /**